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Home , Dunsink Observatory, Johannes Kepler, Robert Stawell Ball

Great : Ball, Robert Stawell

Published: 1907 Categorie(s): Non-Fiction, Biography & autobiography, & Technology, Science and Technics, Science, Astro- nomy Source: The http://archive.org/details/ greatastronomers00balluoft

1 About Ball: Sir , Fellow of The Royal Society, (1 July 1840, – 25 November 1913, Cambridge) was an Irish as- tronomer. He worked for Lord Rosse from 1865 to 1867. In 1867 he became Professor of Applied at the Royal College of Science in Dublin. In 1874 Ball was appointed Royal of Ireland and Andrews Professor of in the University of Dublin at .[1] In 1892 he was appointed Lowndean Professor of Astronomy and Geo- metry at Cambridge University at the same becoming dir- ector of the Cambridge Observatory. His lectures, articles, and books (e.g. Starland and The Story of the Heavens) were mostly popular and simple in style. However, he also published books on mathematical astronomy such as A Treatise on Spher- ical Astronomy. His main interest was mathematics and he de- voted much of his spare time to his "Screw theory". He served for a time as President of the Society. His The Story of the Heavens is mentioned in the "Lestrygonians" chapter of James Joyce's . He was the son of naturalist Robert Ball and Amelia Gresley Hellicar. (Biography from Wik- pedia: http://en.wikipedia.org/wiki/Robert_Stawell_Ball)

Also available on Feedbooks for Ball: • Great Astronomers: (1907) • Great Astronomers: (1907) • Great Astronomers: Johannes (1907) • Great Astronomers: (1907) • Great Astronomers: (1907) • Great Astronomers: Pierre-Simon Laplace (1907) • Great Astronomers: William Herschel (1907) • Great Astronomers: (1907) • Great Astronomers: (1907) • Great Astronomers: John Flamsteed (1907)

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2 Chapter 1

IT was just a year after the death of Galileo, that an infant came into the world who was christened Isaac Newton. Even the great fame of Galileo himself must be relegated to a second place in comparison with that of the philosopher who first ex- pounded the true theory of the . Isaac Newton was born on the 25th of December (old style), 1642, at Woolsthorpe,1 in Lincolnshire, about a half-mile from Colsterworth, and eight miles south of Grantham. His father, Mr. Isaac Newton, had died a few months after his marriage to Harriet Ayscough, the daughter of Mr. James Ayscough, of Market Overton, in Rutlandshire. The little Isaac was at first so excessively frail and weakly that his life was despaired of. The watchful mother, however, tended her delicate child with such that he seems to have thriven better than might have been expected from the circumstances of his infancy, and he ultimately acquired a frame strong enough to outlast the ordin- ary span of human life. For three years they continued to live at Woolsthorpe, the widow's means of livelihood being supplemented by the income from another small estate at Sewstern, in a neighbouring part of Leicestershire. In 1645, Mrs. Newton took as a second hus- band the Rev. Barnabas Smith, and on moving to her new home, about a mile from Woolsthorpe, she entrusted little Isaac to her mother, Mrs. Ayscough. In due time we find that the boy was sent to the public school at Grantham, the name of the master being Stokes. For the purpose of being near his work, the embryo philosopher was boarded at the house of Mr. Clark, an apothecary at Grantham. We learn from Newton him- self that at first he had a very low place in the class lists of the school, and was by no means one of those model school-boys

1.England

3 who find favour in the eyes of the school-master by attention to Latin grammar. Isaac's first incentive to diligent study seems to have been derived from the circumstance that he was severely kicked by one of the boys who was above him in the class. This indignity had the effect of stimulating young Newton's activity to such an extent that he not only attained the desired object of passing over the head of the boy who had maltreated him, but continued to rise until he became the head of the school. The play-hours of the great philosopher were devoted to pur- suits very different from those of most school-boys. His chief amusement was found in making mechanical toys and various ingenious contrivances. He watched day by day with great in- terest the workmen engaged in constructing a windmill in the neighbourhood of the school, the result of which was that the boy made a working model of the windmill and of its ma- chinery, which seems to have been much admired, as indicat- ing his aptitude for . We are told that Isaac also in- dulged in somewhat higher flights of mechanical enterprise. He constructed a carriage, the wheels of which were to be driven by the hands of the occupant, while the first philosoph- ical instrument he made was a clock, which was actuated by water. He also devoted much attention to the construction of paper kites, and his skill in this respect was highly appreciated by his schoolfellows. Like a true philosopher, even at this stage he experimented on the best methods of attaching the string, and on the proportions which the tail ought to have. He also made lanthorns2 of paper to provide himself with as he walked to school in the dark winter mornings. The only love affair in Newton's life appears to have com- menced while he was still of tender years. The incidents are thus described in Brewster's " Life of Newton," a work to which I am much indebted in this chapter.

" In the house where he lodged there were some female inmates, in whose company he appears to have taken much pleasure. One of these, a Miss Storey, sister to Dr. Storey, a physician at Buckminster, near Colsterworth, was two or three years younger than Newton, and to

2.Lanthorn: an archaic term for a lantern.

4 great personal attractions she seems to have added more than the usual allotment of female talent. The soci- ety of this young lady and her companions was always preferred to that of his own schoolfellows, and it was one of his most agreeable occupations to construct for them little tables and cupboards, and other utensils for hold- ing their dolls and their trinkets. He had lived nearly six years in the same house with Miss Storey, and there is to believe that their youthful friendship gradually rose to a higher passion; but the smallness of her por- tion, and the inadequacy of his own fortune, appear to have prevented the consummation of their happiness. Miss Storey was afterwards twice married, and under the name of Mrs. Vincent, Dr. Stukeley visited her at Gr- antham in 1727, at the age of eighty-two, and obtained from her many particulars respecting the early history of our author. Newton's esteem for her continued unabated during his life. He regularly visited her when he went to Lincolnshire, and never failed to relieve her from little pecuniary difficulties which seem to have beset her family."

The schoolboy at Grantham was only fourteen years of age when his mother became a widow for the second time. She then returned to the old family home at Woolsthorpe, bringing with her the three children of her second marriage. Her means appear to have been somewhat scanty, and it was consequently thought necessary to recall Isaac from the school. His recently- born industry had been such that he had already made good progress in his studies, and his mother hoped that he would now lay aside his books, and those silent meditations to which, even at this early age, he had become addicted. It was expected that, instead of such pursuits, which were deemed quite useless, the boy would enter busily into the duties of the farm and the details of a country life. But before long it be- came manifest that the study of nature and the pursuit of knowledge had such a fascination for the youth that he could give little attention to aught else. It was plain that he would make but an indifferent farmer. He greatly preferred experi- menting on his water-wheels to looking after labourers, while

5 he found that working at mathematics behind a hedge was much more interesting than chaffering about the price of bul- locks in the marketplace. Fortunately for humanity his mother, like a wise woman, determined to let her boy's genius have the scope which it required. He was accordingly sent back to Gr- antham school, with the object of being trained in the know- ledge which would fit him for entering the .

6 Chapter 2

It was the 5th of June, 1660, when Isaac Newton, a youth of eighteen, was enrolled as an undergraduate of Trinity College, Cambridge. Little did those who sent him there dream that this boy was destined to be the most illustrious student who ever entered the portals of that great seat of learning. Little could the youth himself have foreseen that the rooms near the gate- way which he occupied would acquire a celebrity from the fact that he dwelt in them, or that the ante-chapel of his college was in good time to be adorned by that noble statue, which is regarded as one of the chief art treasures of Cambridge University, both on account of its intrinsic beauty and the fact that it commemorates the fame of her most distinguished alum- nus, Isaac Newton, the immortal astronomer. Indeed, his ad- vent at the University seemed to have been by no means auspi- cious or brilliant. His birth was, as we have seen, comparat- ively obscure, and though he had already given indication of his capacity for reflecting on philosophical matters, yet he seems to have been but ill-equipped with the routine know- ledge which youths are generally expected to take with them to the Universities. From the outset of his college career, Newton's attention seems to have been mainly directed to mathematics. Here he began to give evidence of that marvellous insight into the deep secrets of nature which more than a century later led so dis- passionate a judge as Laplace to pronounce Newton's immortal work as pre-eminent above all the productions of the human in- tellect. But though Newton was one of the very greatest math- ematicians that ever lived, he was never a for the mere sake of mathematics. He employed his mathematics as an instrument for discovering the laws of nature. His in- dustry and genius soon brought him under the notice of the University authorities. It is stated in the University records

7 that he obtained a Scholarship in 1664. Two years later we find that Newton, as well as many residents in the University, had to leave Cambridge temporarily on account of the breaking out of the plague. The philosopher retired for a to his old home at Woolsthorpe, and there he remained until he was ap- pointed a Fellow of Trinity College, Cambridge, in 1667. From this time onwards, Newton's reputation as a mathematician and as a natural philosopher steadily advanced, so that in 1669, while still but twenty-seven years of age, he was appoin- ted to the distinguished position of Lucasian Professor of Mathematics at Cambridge. Here he found the opportunity to continue and develop that marvellous career of discovery which formed his life's work. The earliest of Newton's great achievements in natural philo- sophy was his detection of the composite character of light. That a beam of ordinary sunlight is, in fact, a mixture of a very great number of different-coloured , is a doctrine now fa- miliar to every one who has the slightest education in physical science. We must, however, remember that this discovery was really a tremendous advance in knowledge at the time when Newton announced it. We here give the little diagram3 originally drawn by Newton, to explain the experiment by which he first learned the com- position of light. A sunbeam is admitted into a darkened room through an opening, H, in a shutter. This beam when not in- terfered with will travel in a straight line to the screen, and there reproduce a bright spot of the same shape as the hole in the shutter. If, however, a prism of glass, A B C, be introduced so that the beam traverse it, then it will be seen at once that the light is deflected from its original track. There is, however, a further and most important change which takes place. The spot of light is not alone removed to another part of the screen, but it becomes spread out into a long-band beautifully col- oured, and exhibiting the hues of the rainbow. At the top are the violet rays, and then in descending order we have the in- digo, blue, green, yellow, orange, and red.

3.Editor's note: The illustrations have been omitted from this eBook. Con- sult the original source for the images. See the attributions section of this eBook.

8 The circumstance in this phenomenon which appears to have particularly arrested Newton's attention, was the elongation which the luminous spot underwent in consequence of its pas- sage through the prism. When the prism was absent the spot was nearly circular, but when the prism was introduced the spot was about five as long as it was broad. To ascertain the explanation of this was the first problem to be solved. It seemed natural to suppose that it might be due to the thick- ness of the glass in the prism which the light traversed, or to the angle of incidence at which the light fell upon the prism. He found, however, upon careful trial, that the phenomenon could not be thus accounted for. It was not until after much pa- tient labour that the true explanation dawned upon him. He discovered that though the beam of white light looks so pure and so simple, yet in reality it is composed of differently col- oured lights blended together. These are, of course, indistin- guishable in the compound beam, but they are separated or disentangled, so to speak, by the action of the prism. The rays at the blue end of the spectrum are more powerfully deflected by the action of the glass than are the rays at the red end. Thus, the rays variously coloured red, orange, yellow, green, blue, indigo, violet, are each conducted to a different part of the screen. In this way the prism has the effect of exhibiting the constitution of the composite beam of light. To us this now seems quite obvious, but Newton did not adopt it hastily. With characteristic caution he verified the explanation by many dif- ferent experiments, all of which confirmed his discovery. One of these may be mentioned. He made a hole in the screen at that part on which the violet rays fell. Thus a violet ray was al- lowed to pass through, all the rest of the light being intercep- ted, and on this beam so isolated he was able to try further ex- periments. For instance, when he interposed another prism in its path, he found, as he expected, that it was again deflected, and he measured the amount of the deflection. Again he tried the same experiment with one of the red rays from the oppos- ite end of the coloured band. He allowed it to pass through the same aperture in the screen, and he tested the amount by which the second prism was capable of producing deflection. He thus found, as he had expected to find, that the second prism was more efficacious in bending the violet rays than in

9 bending the red rays. Thus he confirmed the fact that the vari- ous hues of the rainbow were each bent by a prism to a differ- ent extent, violet being acted upon the most, and red the least. Not only did Newton decompose a white beam into its con- stituent colours, but conversely by interposing a second prism with its angle turned upwards, he reunited the different col- ours, and thus reproduced the original beam of white light. In several other ways also he illustrated his famous proposition, which then seemed so startling, that white light was the result of a mixture of all hues of the rainbow. By combining painters' colours in the proper proportion he did not indeed succeed in producing a mixture which would ordinarily be called white, but he obtained a grey pigment. Some of this he put on the floor of his room for comparison with a piece of white paper. He allowed a beam of bright sunlight to fall upon the paper and the mixed colours side by side, and a friend whom he called in for his opinion pronounced that under these circum- stances the mixed colours looked the whiter of the two. By repeated demonstrations Newton thus established his great discovery of the composite character of light. He at once perceived that his researches had an important bearing upon the principles involved in the construction of a . Those who employed the telescope for looking at the , had been long aware of the imperfections which prevented all the various rays from being conducted to the same . But this imperfection had hitherto been erroneously accounted for. It had been supposed that the reason why success had not been attained in the construction of a was due to the fact that the object glass, made as it then was of a single piece, had not been properly shaped. had abundantly demonstrated that a single , if properly figured, must conduct all rays of light to the same focus, provided all rays experienced equal refraction in passing through the glass. Until Newton's discovery of the composition of white light, it had been taken for granted that the several rays in a white beam were equally refrangible. No doubt if this had been the case, a perfect telescope could have been produced by prop- erly shaping the object glass. But when Newton had demon- strated that light was by no means so simple as had been sup- posed, it became obvious that a satisfactory refracting

10 telescope was an impossibility when only a single object lens was employed, however carefully that lens might have been wrought. Such an objective might, no doubt, be made to con- duct any one group of rays of a particular shade to the same fo- cus, but the rays of other colours in the beam of white light must necessarily travel somewhat astray. In this way Newton accounted for a great part of the difficulties which had hitherto beset the attempts to construct a perfect refracting telescope. We now know how these difficulties can be, to a great extent, overcome, by employing for the objective a composite lens made of two pieces of glass possessing different qualities. To these achromatic object , as they are called, the great development of astronomical knowledge, since Newton's time, is due. But it must be remarked that, although the theoretical possibility of constructing an achromatic lens was investigated by Newton, he certainly came to the conclusion that the diffi- culty could not be removed by employing a composite objective with two different kinds of glass. In this his marvellous saga- city in the interpretation of nature seems for once to have deserted him. We can, however, hardly regret that Newton failed to discover the achromatic objective, when we observe that it was in consequence of his deeming an achromatic ob- jective to be impossible that he was led to the invention of the reflecting telescope. Finding, as he believed, that the defects of the telescope could not be remedied by any application of the principle of re- fraction, he was led to look in quite a different direction for the improvement of the tool on which the advancement of astro- nomy depended. The refraction of light depended, as he had found, upon the colour of the light. The laws of reflection were, however, quite independent of the colour. Whether rays be red or green, blue or yellow, they are all reflected in precisely the same manner from a mirror. Accordingly, Newton perceived that if he could construct a telescope the action of which de- pended upon reflection, instead of upon refraction, the diffi- culty which had hitherto proved an insuperable obstacle to the improvement of the instrument would be evaded. For this purpose Newton fashioned a concave mirror from a mixture of copper and tin, a combination which gives a surface with almost the lustre of silver. When the light of a star fell

11 upon the surface, an image of the star was produced in the fo- cus of this mirror, and then this image was examined by a magnifying eye-piece. Such is the principle of the famous re- flecting telescope which bears the name of Newton.4 The little reflector which he constructed, represented in the adjoining figure, is still preserved as one of the treasures of the Royal Society. The telescope tube had the very modest dimension of one inch in diameter. It was, however, the precursor of a whole series of magnificent instruments, each outstripping the other in magnitude, until at last the culminating point was attained in 1845, by the construction of Lord Rosse's mammoth reflect- or of six feet in aperture.5 Newton's discovery of the composition of light led to an em- bittered controversy, which caused no little worry to the great philosopher. Some of those who attacked him enjoyed consid- erable and, it must be admitted, even well-merited repute in the ranks of science. They alleged, however, that the elonga- tion of the coloured band which Newton had noticed was due to this, to that, or to the other—to anything, in fact, rather than to the true cause which Newton assigned. With characteristic patience and love of truth, Newton steadily replied to each such attack. He showed most completely how utterly his ad- versaries had misunderstood the subject, and how slight in- deed was their acquaintance with the natural phenomenon in question. In reply to each point raised, he was ever able to cite fresh experiments and adduce fresh illustrations, until at last his opponents retired worsted from the combat.

4.Newtonian telescope. 5.Leviathan of Parsonstown is the unofficial name of the Rosse six foot telescope. This is a historic reflecting telescope of 72 in (1.8 m) aperture, which was the largest telescope in the world from 1845 until the con- struction of the 100 in (2.5 m) Hooker Telescope in 1917. The Rosse six foot telescope was built by William Parsons, 3rd Earl of Rosse on his es- tate, Birr Castle, at Parsonstown (now Birr in County Offaly, Ireland). (ht- tp://en.wikipedia.org/wiki/Leviathan_of_Parsonstown)

12 Chapter 3

It has been often a matter for surprise that Newton, throughout his whole career, should have taken so much trouble to expose the errors of those who attacked his views. He used even to do this when it plainly appeared that his ad- versaries did not understand the subject they were discussing. A philosopher might have said, " I know I am right, and wheth- er others think I am right or not may be a matter of concern to them, but it is certainly not a matter about which I need trouble. If after having been told the truth they elect to remain in error, so much the worse for them; my time can be better employed than in seeking to put such people right." This, however, was not Newton's method. He spent much valuable time in overthrowing objections which were often of a very fu- tile description. Indeed, he suffered a great deal of annoyance from the persistency, and in some cases one might almost say from the rancour, of the attacks which were made upon him. Unfortunately for himself, he did not possess that capacity for sublime indifference to what men may say, which is often the happy possession of intellects greatly inferior to his. The subject of still continuing to engross Newton's at- tention, he followed up his researches into the structure of the sunbeam by many other valuable investigations in connection with light. Every one has noticed the beautiful colours manifes- ted in a soap-bubble. Here was a subject which not unnaturally attracted the attention of one who had expounded the colours of the spectrum with such success. He perceived that similar hues were produced by other thin plates of transparent materi- al besides soap-bubbles, and his ingenuity was sufficient to de- vise a method by which the thicknesses of the different films could be measured. We can hardly, indeed, say that a like suc- cess attended his interpretation of these phenomena to that which had been so conspicuous in his explanation of the

13 spectrum. It implies no disparagement to the sublime genius of Newton to admit that the doctrines he put forth as to the causes of the colours in the soap-bubbles can be no longer ac- cepted. We must remember that Newton was a pioneer in ac- counting for the physical properties of light. The facts that he established are indeed unquestionable, but the explanations which he was led to offer of some of them are seen to be unten- able in the fuller light of our present knowledge. Had Newton done nothing beyond making his wonderful dis- coveries in light, his fame would have gone down to posterity as one of the greatest of Nature's interpreters. But it was re- served for him to accomplish other discoveries, which have pushed even his analysis of the sunbeam into the background ; it is he who has expounded the system of the universe by the discovery of the law of universal gravitation. The age had indeed become ripe for the advent of the genius of Newton. Kepler had discovered with marvellous penetration the laws which govern the movements of the around the , and in various directions it had been more or less vaguely felt that the explanation of Kepler's laws, as well as of many other phenomena, must be sought for in connection with the attractive of matter. But the mathematical analysis which alone could deal with this subject was wanting; it had to be created by Newton. At Woolsthorpe, in the year 1666, Newton's attention ap- pears to have been concentrated upon the subject of gravita- tion. Whatever may be the extent to which we accept the more or less mythical story as to how the fall of an apple first direc- ted the attention of the philosopher to the fact that gravitation must extend through , it seems, at all events, certain that this is an excellent illustration of the line of reasoning which he followed. He argued in this way. The attracts the apple ; it would do so, no matter how high might be the tree from which that apple fell. It would then seem to follow that this power which resides in the earth by which it can draw all ex- ternal bodies towards it, extends far beyond the altitude of the loftiest tree. Indeed, we seem to find no limit to it. At the greatest elevation that has ever been attained, the attractive power of the earth is still exerted, and though we cannot by any actual experiment reach an altitude more than a few miles

14 above the earth, yet it is certain that gravitation would extend to elevations far greater. It is plain, thought Newton, that an apple let fall from a point a hundred miles above this earth's surface, would be drawn down by the attraction, and would continually gather fresh until it reached the ground. From a hundred miles it was natural to think of what would happen at a thousand miles, or at hundreds of thousands of miles. No doubt the intensity of the attraction becomes weaker with every increase in the altitude, but that action would still exist to some extent, however lofty might be the elevation which had been attained. It then occurred to Newton, that though the is at a dis- tance of two hundred and forty thousand miles from the earth, yet the attractive power of the earth must extend to the moon. He was particularly led to think of the moon in this connection, not only because the moon is so much closer to the earth than are any of the other celestial bodies, but also because the moon is an appendage to the earth, always revolving around it. The moon is certainly attracted to the earth, and yet the moon does not fall down ; how is this to be accounted for ? The ex- planation was to be found in the character of the moon's present . If the moon were left for a at rest, there can be no doubt that the attraction to the earth would begin to draw the lunar globe in towards our globe. In the course of a few days our satellite would come down on the earth with a most fearful crash. This catastrophe is averted by the circumstance that the moon has a movement of revolution around the earth. Newton was able to calculate from the known laws of mechanics, which he had himself been mainly instrumental in discovering, what the attractive power of the earth must be, so that the moon shall move precisely as we find it to move. It then appeared that the very power which makes an apple fall at the earth's surface, is the power which guides the moon in its . Once this step had been taken, the whole scheme of the uni- verse might almost be said to have become unrolled before the eye of the philosopher. It was natural to suppose that just as the moon was guided and controlled by the attraction of the earth, so the earth itself, in the course of its great annual pro- gress, should guided and controlled by the supreme attractive

15 power of the sun. If this were so with regard to the earth, then it would be impossible to doubt that in the same way the move- ments of the planets could be explained to be consequences of solar attraction. It was at this point that the great laws of Kepler became es- pecially significant. Kepler had shown how each of the planets revolves in an around the sun, which is situated on one of the foci. This discovery had been arrived at from the inter- pretation of observations. Kepler had himself assigned no reas- on why the orbit of a should be an ellipse rather than any other of the infinite number of closed which might be traced around the sun. Kepler had also shown, and here again he was merely deducing the results from observation, that when the movements of two planets were compared to- gether, the squares of the periodic times in which each planet revolved were proportional to the of their mean dis- tances from the sun. This also Kepler merely knew to be true as a fact, he gave no demonstration of the reason why nature should have adopted this particular relation between the dis- tance and the periodic time rather than any other. Then, too, there was the law by which Kepler, with unparalleled ingenu- ity, explained the way in which the velocity of a planet varies at the different points of its track, when he showed how the line drawn from the sun to the planet described equal areas around the sun in equal times. These were the materials with which Newton set to work. He proposed to infer from these the actual laws regulating the by which the sun guides the planets. Here it was that his sublime mathematical genius came into play. Step by step Newton advanced until he had completely accounted for all the phenomena. In the first place, he showed that as the planet describes equal areas in equal times about the sun, the attractive force which the sun exerts upon it must necessarily be directed in a straight line towards the sun itself. He also demonstrated the converse truth, that whatever be the nature of the force which emanated from a sun, yet so long as that force was directed through the sun's centre, any body which revolved around it must describe equal areas in equal times, and this it must do, whatever be the actual character of the law according to which the intensity of the force varies at different parts of the

16 planet's journey. Thus the first advance was taken in the expos- ition of the scheme of the universe. The next step was to determine the law according to which the force thus proved to reside in the sun varied with the dis- tance of the planet. Newton presently showed by a most su- perb effort of mathematical reasoning, that if the orbit of a planet were an ellipse and if the sun were at one of the foci of that ellipse, the intensity of the attractive force must vary in- versely as the square of the planet's distance. If the law had any other expression than the inverse square of the distance, then the orbit which the planet must follow would not be an el- lipse ; or if an ellipse, it would, at all events, not have the sun in the focus. Hence he was able to show from Kepler's laws alone that the force which guided the planets was an attractive power emanating from the sun, and that the intensity of this at- tractive power varied with the inverse square of the distance between the two bodies. These circumstances being known, it was then easy to show that the last of Kepler's three laws must necessarily follow. If a number of planets were revolving around the sun, then suppos- ing the materials of all these bodies were equally affected by gravitation, it can be demonstrated that the square of the peri- odic time in which each planet completes its orbit is propor- tional to the of the greatest diameter in that orbit. These superb discoveries were, however, but the starting- point from which Newton entered on a series of researches, which disclosed many of the profoundest secrets in the scheme of . His natural insight showed that not only large like the sun and the earth, and the moon, attract each other, but that every particle in the universe must attract every other particle with a force which varies inversely as the square of the distance between them. If, for example, the two particles were placed twice as far apart, then the intensity of the force which sought to bring them together would be re- duced to one-fourth. If two particles, originally ten miles asun- der, attracted each other with a certain force, then, when the distance was reduced to one mile, the intensity of the attrac- tion between the two particles would be increased one- hundred-fold. This fertile principle extends throughout the whole of nature. In some cases, however, the calculation of its

17 effect upon the actual problems of nature would be hardly pos- sible, were it not for another discovery which Newton's genius enabled him to accomplish. In the case of two globes like the earth and the moon, we must remember that we are dealing not with particles, but with two mighty masses of matter, each composed of innumerable myriads of particles. Every particle in the earth does attract every particle in the moon with a force which varies inversely as the square of their distance. The calculation of such attrac- tions is rendered feasible by the following principle. Assuming that the earth consists of materials symmetrically arranged in shells of varying densities, we may then, in calculating its at- traction, regard the whole of the globe as concentrated at its centre. Similarly we may regard the moon as concentrated at the centre of its mass. In this way the earth and the moon can both be regarded as particles in point of size, each particle having, however, the entire mass of the corresponding globe. The attraction of one particle for another is a much more simple matter to investigate than the attraction of the myriad different points of the earth upon the myriad different points of the moon. Many great discoveries now crowded in upon Newton. He first of all gave the explanation of the that ebb and flow around our shores. Even in the earliest times the tides had been shown to be related to the moon. It was noticed that the tides were specially high during full moon or during new moon, and this circumstance obviously pointed to the existence of some connection between the moon and these movements of the water, though as to what that connection was no one had any accurate conception until Newton announced the law of gravitation. Newton then made it plain that the rise and fall of the water was simply a consequence of the attractive power which the moon exerted upon the oceans lying upon our globe. He showed also that to a certain extent the sun produces tides, and he was able to explain how it was that when the sun and the moon both conspire, the joint result was to produce espe- cially high tides, which we call " spring tides " ; whereas if the solar was low, while the lunar tide was high, then we had the phenomenon of "neap" tides.

18 But perhaps the most signal of Newton's applications of the law of gravitation was connected with certain irregularities in the movements of the moon. In its orbit round the earth our satellite is, of course, mainly guided by the great attraction of our globe. If there were no other body in the universe, then the centre of the moon must necessarily perform an ellipse, and the centre of the earth would lie in the focus of that ellipse. Nature, however, does not allow the movements to possess the simplicity which this arrangement would imply, for the sun is present as a source of disturbance. The sun attracts the moon, and the sun attracts the earth, but in different degrees, and the consequence is that the moon's movement with regard to the earth is seriously affected by the influence of the sun. It is not allowed to move exactly in an ellipse, nor is the earth exactly in the focus. How great was Newton's achievement in the solution of this problem will be appreciated if we realise that he not only had to determine from the law of gravitation the nature of the disturbance of the moon, but he had actually to construct the mathematical tools by which alone such calculations could be effected. The resources of Newton's genius seemed, however, to prove equal to almost any demand that could be made upon it. He saw that each planet must disturb the other, and in that way he was able to render a satisfactory account of certain phenomena which had perplexed all preceding investigators. That mysteri- ous movement by which the pole of the earth sways about among the stars had been long an unsolved enigma, but New- ton showed that the moon grasped with its attraction the pro- tuberant mass at the equatorial regions of the earth, and thus tilted the earth's axis in a way that accounted for the phe- nomenon which had been known but had never been explained for two thousand years. All these discoveries were brought to- gether in that immortal work, Newton's "Principia."

19 Chapter 4

Down to the year 1687, when the "Principia" was published, Newton had lived the life of a recluse at Cambridge, being en- tirely occupied with those transcendent researches to which we have referred. But in that year he issued from his seclusion under circumstances of considerable historical interest. King James the Second attempted an invasion of the rights and priv- ileges of the University of Cambridge by issuing a command that Father Francis, a Benedictine monk, should be received as a Master of Arts in the University, without having taken the oaths of allegiance and supremacy. With this arbitrary com- mand the University sternly refused to comply. The Vice-Chan- cellor was accordingly summoned to answer for an act of con- tempt to the authority of the Crown. Newton was one of nine delegates who were chosen to defend the independence of the University before the High Court. They were able to show that Charles the Second, who had issued a mandamus under some- what similar circumstances, had been induced after due con- sideration to withdraw it. This argument appeared satisfactory, and the University gained their case. Newton's next step in public life was his election, by a narrow majority, as member for the University, and during the years 1688 and 1689, he seems to have attended to his parliamentary duties with con- siderable regularity. An incident which happened in 1692 was apparently the cause of considerable disturbance in Newton's equanimity, if not in his health. He had gone to early morning chapel, leaving a lighted candle among his papers on his desk. Tradition as- serts that his little dog "Diamond" upset the candle; at all events, when Newton came back he found that many valuable papers had perished in a conflagration. The loss of these manuscripts seems to have had a serious effect. Indeed, it has been asserted that the distress reduced Newton to a state of

20 mental for a considerable time. This has, appar- ently, not been confirmed, but there is no doubt that he experi- enced considerable disquiet, for in writing on September 13th, 1693, to Mr. Pepys, he says: " I am extremely troubled at the embroilment I am in, and have neither ate nor slept well this twelvemonth, nor have my former consistency of mind." Notwithstanding the fame which Newton had achieved by the publication of his "Principia," and by all his researches, the State had not as yet taken any notice whatever of the most il- lustrious man of science that this or any other country has ever produced. Many of his friends had exerted themselves to pro- cure him some permanent appointment, but without success. It happened, however, that Mr. Montagu, who had sat with New- ton in Parliament, was appointed Chancellor of the Exchequer in 1694. Ambitious of distinction in his new office, Mr. Montagu addressed himself to the improvement of the current coin, which was then in a very debased condition. It fortunately happened that an opportunity occurred of appointing a new of- ficial in the Mint; and Mr. Montagu on the 19th of March, 1695, wrote to offer Mr. Newton the position of warden. The salary was to be five or six hundred a year, and the business would not require more attendance than Newton could spare. The Lucasian professor accepted this post, and forthwith entered upon his new duties. The knowledge of physics which Newton had acquired by his experiments was of much use in connection with his duties at the Mint. He carried out the re-coinage with great skill in the course of two years, and as a reward for his exertions, he was appointed, in 1697, to the Mastership of the Mint, with a salary between £1,200 and £1,500 per annum. In 1701 his duties at the Mint being so engrossing, he resigned his Lucasian pro- fessorship at Cambridge, and at the same time he had to sur- render his fellowship at Trinity College. This closed his connec- tion with the University of Cambridge. It should, however, be remarked that at a somewhat earlier stage in his career he was very nearly being appointed to an office which might have en- abled the University to retain the great philosopher within its precincts. Some of his friends had almost succeeded in secur- ing his nomination to the Provostship of King's College,

21 Cambridge ; the appointment, however, fell through, inasmuch as the statute could not be evaded, which required that the Provost of King's College should be in holy orders. In those days it was often the custom for illustrious mathem- aticians, when they had discovered a solution for some new and striking problem, to publish that problem as a challenge to the world, while withholding their own solution. A famous in- stance of this is found in what is known as the Brachistochrone problem, which was solved by John Bernouilli. The nature of this problem may be mentioned. It was to find the shape of the along which a body would slide down from one point (A) to another point (B) in the shortest time. It might at first be thought that the straight line from A to B, as it is undoubtedly the shortest distance between the points, would also be the path of quickest descent; but this is not so. There is a curved line, down which a bead, let us say, would run on a smooth wire from A to B in a shorter time than the same bead would require to run down the straight wire. Bernouilli's problem was to find out what that curve must be. Newton solved it correctly; he showed that the curve was a part of what is termed a cyc- loid—that is to say, a curve like that which is described by a point on the rim of a carriage-wheel as the wheel runs along the ground. Such was Newton's geometrical insight that he was able to transmit a solution of the problem on the day after he had received it, to the President of the Royal Society. In 1703 Newton, whose world-wide fame was now estab- lished, was elected President of the Royal Society. Year after year he was re-elected to this distinguished position, and his tenure, which lasted twenty-five years, only terminated with his life. It was in discharge of his duties as President of the Royal Society that Newton was brought into contact with Prince George of Denmark. In April, 1705, the Queen paid a visit to Cambridge as the guest of Dr. Bentley, the then Master of Trinity, and in a court held at Trinity Lodge on April 15th, 1705, the honour of knighthood was conferred upon the dis- coverer of gravitation. Urged by illustrious friends, who sought the promotion of knowledge, Newton gave his attention to the publication of a new edition of the "Principia." His duties at the Mint, however, added to the supreme duty of carrying on his original

22 investigations, left him but little time for the more ordinary task of the revision. He was accordingly induced to associate with himself for this purpose a distinguished young mathem- atician, Roger Coates, a Fellow of Trinity College, Cambridge, who had recently been appointed Plumian Professdr of Astro- nomy. On July 27th, 1713, Newton, by this time a favourite at Court, waited on the Queen, and presented her with a copy of the new edition of the " Principia." Throughout his life Newton appears to have been greatly in- terested in theological studies, and he specially devoted his at- tention to the subject of prophecy. He left behind him a manuscript on the prophecies of Daniel and the Apocalypse of St. John, and he also wrote various theological papers. Many other subjects had from time to time engaged his attention. He studied the laws of heat; he experimented in pursuit of the dreams of the Alchymist; while the philosopher who had re- vealed the mechanism of the heavens found occasional relaxa- tion in trying to interpret hieroglyphics. In the last few years of his life he bore with fortitude a painful ailment, and on Monday, March 20th, 1727, he died in the eighty-fifth year of his age. On Tuesday, March 28th, he was buried in Westmin- ster Abbey. Though Newton lived long enough to receive the honour that his astonishing discoveries so justly merited, and though for many years of his life his renown was much greater than that of any of his contemporaries, yet it is not too much to say that, in the years which have since elapsed, Newton's fame has been ever steadily advancing, so that it never stood higher than it does at this moment. We hardly know whether to admire more the sublime discov- eries at which he arrived, or the extraordinary character of the intellectual processes by which those discoveries were reached. Viewed from either standpoint, Newton's " Principia " is incomparably the greatest work on science that has ever yet been produced.

23 Attributions

The cover image is from http://commons.wikimedia.org/wiki/ File:GodfreyKneller-IsaacNewton-1689.jpg in the public do- main. This image is in the public domain in the United States, and those countries with a copyright term of life of the author plus 100 years or less. The text was taken from Great Astronomers, by Sir Robert Stawell Ball (1840-1913), second edition(1907), at http://archive.org/details/greatastronomers00balluoft .

24 www.feedbooks.com Food for the mind

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